In recent years, among radiation therapy systems for the purpose of cancer therapy, there have been advanced development and/or construction of a cancer therapy system that utilizes a particle beam of proton, heavy ion, etc. (called, in particular, as a particle beam therapy system) As is well known, according to a particle beam therapy utilizing the particle beam, a cancer diseased site can be irradiated in a concentrated manner as compared to the conventional radiation therapy utilizing an X-ray, a γ-ray, etc. and thus it is possible to perform the therapy without affecting normal cells.
Charged particles formed into a beam (referred to also as a charged particle beam or the particle beam) that is obtained by circularly accelerating the charged particles by an accelerator (circular accelerator) such as a synchrotron, etc. and taking out the charged particles (mostly, protons or carbon ions) accelerated up to a high energy from the circular trajectory thereof, are transported using a beam transport system so as to be applied to a physical experiment in which an intended object is irradiated therewith or a particle beam therapy such as a cancer therapy, etc. In the cancer therapy by the accelerated charged particles, that is, in the particle beam therapy, in order to keep vital organs away or to prevent normal tissues from being damaged at the time of the therapy, changing a direction of the irradiation is generally performed. In order to irradiate the patient from an arbitrary direction, a particle beam rotational irradiation apparatus is provided in many cases. The particle beam rotational irradiation apparatus (called also as a rotational irradiation apparatus, when appropriate) is provided with an irradiation nozzle that is mounted on a rotary gantry and radiates the particle beam. The rotary gantry is configured to rotate the irradiation nozzle that radiates the particle beam so that the irradiation nozzle can radiate the charged particle beam to the patient from an arbitrary rotation angle.
In the case where the irradiation nozzle is rotated so that it can radiate from an arbitrary angle to the patient, a treatment table on which the patient is secured is required to be fixed whereas the irradiation nozzle is rotating, so that the treatment table results in a configuration that protrudes from the side of a stationary portion provided in the building side. Thus, although an access floor is provided for allowing a doctor, a radiological technologist or the like who performs the therapy to always approach the patient so as to perform works, the access floor is required to always keep a function as a floor, regardless of the rotation angle of the rotary gantry.
Although the access floor is required as described above, because the access floor interferes with a passing area of the irradiation nozzle mounted on the rotary gantry, such an interfering area between the irradiation nozzle and the access floor has to be evacuated off during passing of the irradiation nozzle. Thus, it is necessary to provide an access floor that can be retracted only when the irradiation nozzle is going to pass through the access floor (hereinafter, referred to as a movable floor) and an access floor that is fixed.
For example, in Patent Document 1, there is described a particle beam rotational irradiation apparatus provided with an openable floor. FIG. 7 is a diagram showing the conventional-type particle beam rotational irradiation apparatus. The particle beam rotational irradiation apparatus placed in a building 106 includes a frame 101, a rotary ring 102, a rotation driving device 103, a gantry roller 104, a braking device 105, a beam transport instrument 107, an irradiation nozzle 108, a treatment table 109, a movable floor 110 and an access floor 115. The frame 101, the rotary ring 102, the rotation driving device 103 and the gantry roller 104 are coupled together, so that the frame 101 rotates as the rotation driving device 103 rotates. When the irradiation nozzle 108 becomes close, by its rotation, to the movable floor 110, divided movable floors that form the movable floor 110 are going to be retracted one by one in the direction of the rotation axis. With the progress of rotation of the irradiation nozzle 108, the movable floor 110 is retracted in the direction of the rotation axis.
Meanwhile, in Patent Document 2, there is described, as another type of particle beam rotational irradiation apparatus, a particle beam rotational irradiation apparatus of a so-called corkscrew type. In this corkscrew-typo particle beam rotational irradiation apparatus, charged particle beam is guided in such a manner that the charged particle beam is once deflected using two bending magnets so that its beam transport line becomes perpendicular to the rotation axis of the rotary gantry, and is thereafter deflected again using two bending magnets so that the charged particle beam becomes directed to an isocenter (intersection point between the gantry rotation axis and the beam axis, that is a reference of irradiation target) within a plane perpendicular to the central axis of the gantry. Such a beam transport line allows the rotary gantry to be shorter in its length with respect to the direction of the rotation axis. Note that the bending magnet is usually a two-pole electromagnet that has two poles.